The present invention relates to the field of radiocommunication. It applies especially in radiocommunication systems using polarization diversity.
Conventionally, mobile radiocommunication systems use diversity processing techniques that allow their performance to be improved. Diversity processing is based on the combining of information received from several signals transmitted from a source to a receiver. Diversity may be introduced into several parameters, such as time, space, frequency or polarization of an electromagnetic wave, and this gives rise to many techniques.
Various transmission diversity methods are, for example, currently provided in third-generation cellular networks of the UMTS (Universal Mobile Telecommunications System) type in the downlink direction (from the network to the mobile units). A first category of methods, called open-loop transmission diversity methods, employ STTD (Space-Time Transmit Diversity) or TSTD (Time Switch Transmit Diversity) schemes.
The STTD diversity scheme is based on space-time coding. According to this scheme, two signals s0 and s1 are transmitted simultaneously at a time t and over a period T of a symbol time on two antennas 0 and 1 respectively. At time t+T, the signals −s1* and s0* are transmitted simultaneously over a period T to the antennas 0 and 1 respectively (the symbol “*” denoting the complex conjugation operation). It thus makes it possible, in a system consisting of two transmit antennas and one receive antenna, to obtain the same order of diversity as in a system consisting of one transmit antenna and two receive antennas, from which the signals are processed by a diversity receiver using the optimal combining method (MRC, Maximum Ratio Combining).
The STTD scheme as applied in UMTS-type networks is described in Section 5.3.1.1.1 of the Technical Specification 3G TS 25.211, “Physical channels and mapping of transport channels onto physical channels (FDD)”, Version 3.9.0 published in December 2001 by 3GPP (“3rd Generation Partnership Project”).
Closed-loop transmit diversity is also employed in these third-generation networks. A detailed description of this is given in Section 7 of the Technical Specification 3G TS 25.214, “Physical layer procedures (FDD)—Release 1999”, Version 3.9.0, published in December 2001 by 3GPP.
According to this scheme, a signal is transmitted from two antennas, after it has been weighted in each transmission branch by a weight intended to correct its phase and/or its amplitude so as to maximize the power of the useful signal received by the receiver. A feedback loop is used to update the optimal weight vector at the transmitter. Such a scheme is potentially sensitive to the speed of movement of the receiver. A high speed may require the phase to be corrected and the weighting vector to be updated more rapidly than the speed of the feedback loop currently provided.
The base stations of cellular systems that exploit polarization diversity use, for example, a cross-polar antenna system, i.e. two antennas placed at the same point and arranged at 90° to each other (one is, for example, sensitive to the vertical polarization and the other sensitive to the horizontal polarization). The transmitted signal is received via a polarization-diversity antenna system in two branches of the receiver. Combining techniques are then used to take advantage of the independence of behavior along the propagation path of orthogonally polarized signals. More specifically, the polarization diversity gain results from the rotation of the polarization when the transmitted electromagnetic wave is randomly reflected off obstacles. Conventionally, it is accepted that signals received with polarization diversity must be weakly correlated so that the combining delivers a gain that justifies the use of this technique. Lee and Yeh (“Polarization diversity system for mobile radio”, IEEE Trans. Com., Vol. COM-20, No. 5, pp. 912-922, 1972) have considered that effective diversity may be achieved with a correlation coefficient of less than 0.7.
The present invention relates especially to a dual transmit polarization diversity scheme. In such a scheme, the radio transmission is distributed over two units each designed to transmit a signal in a pre-determined polarization. It may for example be employed in a base station provided with a cross-polar antenna system and with two radio transmitters, one being designed to transmit in vertical linear polarization and the other in horizontal linear polarization.
Such base stations are described for example in U.S. Pat. No. 6,411,824 and WO 01/54230.
Application WO 01/54230 describes in particular a system for reducing the effects of fast fading observed in a communication channel with a mobile unit. According to the method described, a transmitter (of a base station or of a mobile unit) scans predetermined transmission polarization states. An optimal state is selected using an open-loop or closed-loop method. Such a method requires a rate of updating the optimal polarization, on the basis of minimizing the effects of fading, corresponding to the rate of change of this phenomenon. In the example described, the matching is thus carried out at a rate of the order of one frame of 10 ms duration. Such a rate is somewhat incompatible with a closed-loop method, the rate of the feedback loop imposing an excessive load on the air interface, taking into account the advantages afforded by the method.
One object of the present invention is to propose another mode of polarization diversity, which provides an appreciable receive gain without seeking to follow the fast fading of the channel, which would impose a signaling load difficult to accept.